Graphite is a crystalline form of carbon which is a moderate conductor of electricity and is relatively soft. The carbon atoms in graphite form a layered structure in which the carbon atoms in each plane are covalently bonded to each other.
In graphite, each carbon atom is bonded to three other carbon atoms in each layer. After forming a strong covalent sigma (.sigma.) bond with each neighbor, each carbon atom still has one free electron remaining and these are paired up in a system of weak pi (.pi.) bonds. It is the fourth, or pi electron, that is responsible for the laminar structure and electronic properties of graphite. The pi electrons form a delocalized distribution which constitute a valance band for the fundamental state of graphite and a conduction band for the excited stage.
The graphite layers, or sheets, are weakly bonded to each other by van der Waals forces. This weak bonding between layers may account for both the lubrication properties and the highly anisotropic (layer directed) conductive nature of graphite.
The intercalation, or the incorporation of acceptor or donor compounds into the layered structure of graphite has been known for many years. Intercalated graphite contains acceptor or donor compounds (an intercalant) interspaced between the layers or planes of the graphite. Thus, during synthesis the intercalant diffuses in between the planes and electron exchanges take place between the intercalant and the electronic structure of graphite.
Intercalated graphite can form different stage products. A first stage intercalation compound has alternate layers of graphite and the intercalant; a second stage compound has two layers of graphite for each layer of intercalant and a third stage compound has three layers of graphite per layer of intercalant, etc.
Intercalation of graphite with certain intercalants greatly enhances the ability of graphite to conduct electricity in the planes of the layered structure. Some graphite intercalation compounds, for example, conduct electricity in the direction of the planes almost as well as metals. In addition to being an excellent electrical conductor, intercalated graphite is lightweight and relatively inexpensive when compared to many metallic conductors. It is this promise of an inexpensive and lightweight conductor that has provided the motivation for experimentation with intercalated graphite particles. It should be noted that intercalated graphite might be specially tailored for particular conductor and semiconductor uses. Tailoring these compounds could be done by changing the staging of the intercalated graphite, and the type and amount of the intercalant.
The major problem preventing successful use of graphite intercalates as conductors is the thermal and chemical instability of intercalation compounds. Most existing graphite intercalation compounds and virtually all the useful electrically conducting ones are easily decomposed or are very hydroscopic. As a result, most graphite intercalation compounds are unstable in air because the intercalant diffuses out of the graphite lattice at a substantial rate or the compound rapidly exfoliates in contact with water vapor. This instability renders graphite intercalation compounds unsuitable for use as permanent electrical connectors. Because of the instability of graphite intercalations, even structural studies of the compounds must often be conducted under controlled atmospheres or within sealed tubes.
A need therefore exists for air-stable graphite intercalation compounds.